A research team from the Korea Institute of Energy Research (KIER) in South Korea has successfully constructed and demonstrated a small-scale solar seasonal thermal storage system that integrates heat pumps for the purpose of heating greenhouses. The system was analysed over a three-year period at a two-section greenhouse dedicated to cultivating Dekopon oranges on an area of 2,090 m² and Setoka oranges on 1,760 m².
According to Kyoung-Ho Lee, the corresponding author of the study, the research presents the outcomes of a three-year performance evaluation of a greenhouse heating system aimed at lowering heating expenses through the utilisation of solar energy and heat pumps. Unlike traditional large-scale seasonal thermal storage tanks, this study focused on a compact storage tank that is more appropriate for greenhouse applications, which was installed within the greenhouse machine room to minimise thermal losses.
Lee explained that a portion of the solar heat accumulated during the summer months was employed as a heat source for the heat pump, thereby enhancing its efficiency. Additionally, a shallow-ground heat exchanger was incorporated to ensure a consistent heat source.
To facilitate the installation of the equipment, the researchers removed 114 m² of the existing greenhouse structure and constructed a compact mechanical room. On the roof, they installed 96 m² of evacuated-tube solar collectors alongside 108 m² of flat-plate collectors. The system also featured a 350 m³ seasonal storage tank, a 100 m³ buffer tank, a 45 RT dual-source heat pump, two 45 RT air-source heat pumps, along with piping, circulation pumps, and control equipment situated either inside or beneath the structure.
Throughout the heating season, the system provided solar heat directly to the buffer tank. During times when heating was not required, the heat was stored in the seasonal storage tank. High-temperature water from this tank could be transferred to the buffer tank for immediate heating, while water temperatures ranging from 10 °C to 35 °C could be used as a heat source for the dual-source heat pump.
Surplus heat could also be directed to a shallow-ground thermal storage system, which consisted of U-shaped pipes installed vertically to a depth of 5 meters and three tiers of horizontal pipes at a depth of 2 meters. The buffer tank received heat from the solar collectors, seasonal storage tank, and heat pumps, distributing it to the greenhouses via 25 fan-coil units rated at 10,000 kcal/h and 30 units rated at 20,000 kcal/h.
The monitoring of the system took place from April 2022 to May 2025, encompassing three annual cycles. The total annual solar irradiation recorded was 1,958.3 kWh/m² in the first year, 1,744.1 kWh/m² in the second year, and 1,817.2 kWh/m² in the third year.
In December of the first year, the researchers noted monthly average minimum and maximum temperatures of 1.7 °C and 26.7 °C, respectively. In the second year, these values were 3.1 °C and 27.5 °C in November, and in the third year, they recorded 2 °C and 29 °C in January.
The solar collector system achieved an annual average thermal production efficiency of 31.8%. Although the small-capacity seasonal thermal storage tank had some limitations, such as a higher unit cost per volume and lower efficiency compared to larger tanks, the optimised valve configuration and improved insulation design resulted in an annual average thermal storage efficiency of 55.1%.
The multi-source heat pump, which utilised the seasonal heat source from the storage tank, demonstrated an enhanced coefficient of performance (COP) that improved as the water source temperature increased, achieving an annual average COP of 4.1. In instances where the multi-source heat pump was not operational, an air-source heat pump was employed, which yielded an annual average COP of 3.9. Although the shallow-ground thermal storage system exhibited low long-term storage efficiency due to rapid heat loss from suboptimal ground conditions, it showed potential in stabilising the heat pump source temperature, thereby maintaining consistent performance during peak heating periods.
The findings of this research were published in the journal Applied Thermal Engineering under the title “Long-term thermal performance of a heat pump system with a small-scale solar seasonal hot water storage tank for greenhouse heating.”
Source: pv magazine Global
